The present invention relates to a method for producing an optical fiber having reduced hydrogen sensitivity. More particularly, it relates to a method for producing a glass optical fiber having reduced hydrogen attenuation effects in the operating windows in the wavelength ranges 1270-1330 nm and 1520-1580 nm.
The low attenuation and dispersion characteristics of optical fibers have been advantageously employed to form long repeaterless links. In certain instances it is desirable to employ a large percentage of the loss budget made available by the low loss fiber, thereby providing very little safety factor. If, after the fiber is drawn, sufficient attenuation increase occurs at a transmitting wavelength, system operation can be interrupted.
Various kinds of single-mode and multimode optical fibers have been found to exhibit a reversible attenuation increase caused by the permeation of hydrogen gas into the optical fiber after it has been installed. Attenuation increase due to the interaction of hydrogen with the low loss fiber has been known to exist for several years. There are several known hydrogen induced attenuation effects: (1) interstitial hydrogen which is directly proportional to the partial pressure of hydrogen and is reversible; (2) increases in the phosphorous-hydroxyl absorption (1300-2000 nm) which precludes the use of P.sub.2 O.sub.5 as a dopant except in low (less than 0.1 wt. %) concentrations; (3) under high temperature-long time H.sub.2 exposure, there results a high absorption at short wavelengths that has an extensive tail extending through the visible and into the infrared region; and (4) transient absorption that occurs when H.sub.2 first arrives in the fiber core region with most notably peaks at 1530, 1330, 1380, and 1440 nm. Effect (4), referred to herein as the transient hydrogen sensitive attenuation phenomenon, is addressed by the method of this invention. A delayed, hydrogen sensitive attenuation increase can occur from a few hours to many days after the fiber has been manufactured, depending on the temperature and the hydrogen partial pressure to which the fiber is subjected. Immediately after the attenuation increase has reached a maximum, it begins to decrease to a residual value that is about 15% of the maximum attenuation increase over the base level. The attenuation increase may occur after the optical fiber has been installed, thereby creating the possibility that the system will be rendered at least temporarily inoperative.
Additional discussion of hydrogen produced attenuation increase can be found in the following publications:
(a) A. Iino et al. "Mechanisms of Hydrogen-Induced Losses in Silica Based Optical Fibers" OFC'90, TUB3, PA1 (b) Y. Yokomachi et al. "Hydrogen-Induced Absorption Bands in Silica Core Fibers in the Infrared Region" OFC'89 WQ12, PA1 (c) H. Kajioka et al. "Analysis of Drawing-Induced Stress and Loss Mechanisms in Dispersion-Shifted Single-mode Optical Fibers" OFC'88, W13, PA1 (d) M.G. Blankenship et al. "Short Term Transient Attenuations in Single-mode Optical Fibers Due to Hydrogen" WA3, OFC/IOOC'87, PA1 (e) K. Nagasawa et al. "Effect of Cladding Material on 2-eV Optical Absorption in Pure-Silica Core Fibers and Method to Suppress the Absorption" Japanese Journal of Applied Physics, v.26, No. 1, Jan, 1987, pp. 148-151, PA1 (f) H. Bauch et al. "Properties of PICVD-Fibers with Pure SiO.sub.2 -Core: The Influence of the Preform Collapse Process" Journal of Optical Communications, v. 8 (1987) No. 4, pp. 140-142, PA1 (g) P.J. LeMaire et al. "Optical Spectra of Silica Core Optical Fibers Exposed to Hydrogen" Mat. Res. Co. Symp. Proc., vol. 88, PA1 (h) K. Nagasawa et al. "Effect of Oxygen Content on Defect Formation in Pure-Silica Core Fibers" Japanese Journal of Applied Physics, vol. 26, No. 5, May 1987, pp L554-L557, PA1 (i) K. Noguchi et al. "Loss Increase For Optical Fibers Exposed to Hydrogen Atomsphere" Journal of Lightwave Technology, vol. Lt-3, No. 2, April 1985, and PA1 (j) K. Nagasawa et al. "The 1.52 .mu.m Absorption Band Induced by Hydrogen Treatment in Optical Fibers" OFC/IOOC 1987, WA4.
The effect of hydrogen on a particular optical fiber will be described in order to graphically illustrate the transient hydrogen sensitive attenuation phenomenon. Single-mode fibers having cores of silica doped with 20 wt. % GeO.sub.2 (0.96% delta) were studied in order to characterize the transient hydrogen sensitive attenuation phenomenon. Fibers having 1.1 km lengths were placed in room temperature chambers containing 10.sup.-4 atmosphere hydrogen to 1.0 atmosphere hydrogen. A delayed, hydrogen sensitive attenuation increase occurred from a few hours to many days after a fiber had been manufactured, depending on the hydrogen partial pressure to which a fiber was subjected. Since the diffusion of hydrogen to the light propagating region of the affected fiber initiates the attenuation increase, temperature is also a factor. FIG. 1 shows spectral attenuation curves for a fiber that is relatively sensitive to hydrogen. Curve 10 illustrates the spectral attenuation of the fiber immediately after it was drawn. As illustrated by curve 11, the fiber experienced attenuation increases at 1330 nm, 1440 nm and 1530 nm after exposure to 0.1% hydrogen for 25 days. Attenuation increased from about 0.2 dB/km to almost 1 dB/km at 1530 nm. Curve 12 shows that after 153 days, attenuation decreased from its maximum value, but it never decreased to its as-drawn value at 1530 nm. Its residual value at that wavelength is about 0.3 dB/km.
Curves 16 and 17 of FIG. 2 illustrate the manner in which attenuation increases with respect to time for this type of fiber at two of the affected wavelengths, 1530 nm and 1380 nm, respectively. The fiber is drawn at time t.sub.O. Very little attenuation increase above the base value of A.sub.i occurs between times t.sub.O and t.sub.d. Attenuation rapidly increases between times t.sub.d and t.sub.m, at which time a maximum attenuation A.sub.m occurs. The term A.sub.m(1530), for example, refers to the maximum attenuation at 1530 nm. Thereafter, attenuation gradually decreases to a residual value at time t.sub.r. The residual attenuation A.sub.r is about 15% of the maximum attenuation increase A.sub.m.
This transient attenuation increase mechanism has what appears to be a diffusion period of several days to several hundreds of days, depending upon hydrogen concentration and ambient temperature. The diffusion period is given by the following equation: EQU t.sub.d =4.4.times.10.sup.-8 .times.(1+0.015/P).times.e.sup.(5234/T)
where t is time in days, P is partial pressure of hydrogen 25 in atmospheres, and T is temperature in Kelvin.
FIG. 3 schematically illustrates the occurrence of the hydrogen diffusion/reaction process within a cross-section of fiber 20. Defect sites 21, 21' can be located in core 22, cladding 23 or both. It is hypothesized that these defect sites are formed under certain drawing conditions. The concentration of defect sites within the experimental glass fiber samples tested which react with hydrogen was typically less than 100 ppb.
An installed optical fiber is subjected to some source of hydrogen such as air or other atmosphere that surrounds the fiber. Hydrogen molecules 24 diffuse inwardly from the hydrogen source 25 outside the cladding 23. As molecules 24 begin to diffuse inwardly from the cladding surface, they react with defect sites to form reacted sites 21' which are located beyond some radius represented by dashed line 26. Time delay t.sub.d of FIG. 2 results from the diffusion time of the hydrogen molecules along with the reactive consumption of the hydrogen molecules at reactive defect sites in the cladding glass until the hydrogen reaches the light propagating region of the fiber. After the hydrogen molecules have reacted with essentially all of the defect sites at locations of radii beyond the light propagation region of the fiber, they begin to react with sites that are located in the light propagation region, thereby causing an attenuation increase.
The magnitude of the attenuation increase, which appears to be related to the number of defects in the light propagating region of the fiber, has been known to increase the attenuation from a base value less than about 0.2 dB/km to more than 1.0 dB/km at 1530 nm. The maximum transient attenuation lasts for only a few hours at room temperature. The aforementioned Nagasawa et al. OFC/IOOC '87 publication suggests that the attenuation increase is due to a weakened H--H bond when a hydrogen bond is formed with a peroxy radical. The present method should therefore be effective to reduce attenuation in germanate as well as silicate glasses since both of these kinds of glasses contain peroxy radicals.